The present invention relates to a solvent of a chromogenic dye-precursor material for preparing a pressure-sensitive recording paper sheet excellent in the stability in preservation of the colour-developed images and in the colour-developer under severe coldness, a pressure-sensitive recording paper sheet prapared by using the solvent and a process for producing the solvent comprising coupling ethylbenzene and xylene by side-chain dehydrogenation. More in detail, the present invention relates to (1) a solvent of a chromogenic dye-precursor material for preparing a pressure-sensitive recording paper sheet, which the solvent comprises diarylalkanes containing not less than 80% by weight of a mixture of 1-tolyl-2-phenylpropane and 1,2-ditolylethane, obtained by coupling ethylbenzene and xylene by side-chain dehydrogenation, (2) a pressure-sensitive recording paper sheet prepared by using the solvent comprising diarylalkanes of a chromogenic dye-precursor material (colour former), and (3) a process for producing the solvent of a chromogenic dye-precursor material for a pressure-sensitive recording paper sheet, which process comprises coupling ethylbenzene and xylene by side-chain dehydrogenation at a temperature of not less than 100.degree. C. in the presence of a peroxodisulfate.
The mixture of dimers obtained by the liquid-phase coupling method of ethylbenzene and xylene contains 1,2-ditolylethane, 1-tolyl-2-phenylpropane and 2,3-diphenylbutane, and since these dimers are aromatic compounds boiling at higher temperatures, the thus obtained mixture of dimers is useful as an organic thermal medium and an electric insulating oil other than the solvent of the chromogenic dye-precursor material for the pressure-sensitive recording paper sheet.
In general, the pressure-sensitive recording paper sheet is composed of (1) a combination of a paper sheet (hereinafter referred to as "CB paper") on the back side of which microcapsules containing a solution of a colourless electron-donator having a colouring reactivity (hereinafter referred to as "chromogenic dye-precursor material") has been applied and another paper sheet (hereinafter referred to as "CF paper") on the upper side of which a colour-developing agent (hereinafter referred to as "colour-developer") having an ability of forming a coloured product by reacting with the above-mentioned chromogenic dye-precursor material has been applied, (2) a paper sheet (hereinafter referred to as "CFB paper") on one side of which the microcapsules have been coated therewith and on the other side of which the colour-developer has been coated therewith, (3) a combination of the CB paper and CF paper or (4) a paper sheet on one side of which the microcapsules and the colour-developer are coated therewith in the form of layer or a mixture of the microcapsules and the colour-developer are coated therewith. On applying an artificial pressure on every one of the pressure-sensitive recording paper sheet, the microcapsules on the thus pressured part are broken to make the chromogenic dye-precursor material and the colour-developer in contact each other, thereby developing a colour.
In the pressure-sensitive recording paper sheet, the materials which exert an important influence on the quality thereof are the solvent of the chromogenic dye-precursor material contained in the microcapsule and the wall material forming the microcapsule.
Hitherto, the specificities required to the solvent of the chromogenic dye-precursor material for the pressure-sensitive recording paper sheet have been the followings.
(1) The solubility of the chromogenic dye-precursor material in the solvent is high.
(2) The colour-developing rate is high, the concentration of the thus developed colour is high and the stability of the colour tone is also high after the colour-development.
(3) The solvent is stable against lights, heat and chemicals.
(4) The solvent is substantially odorless.
(5) The solvent is harmless to human body and free from danger.
(6) The solvent is favorably biodegradable not to cause environmental pollution.
However, with the propagation of the use of the pressure-sensitive recording paper sheet together with the rapid development of electronics, the qualitative requirements to the developed colour image have come to be severer.
Namely, the pressure-sensitive recording paper sheet have come to be used for recording in the outdoor services such as gas stations in cold districts, and the colour-developed pressure-sensitive recording paper sheet have come to be preserved for a long time as the recorded documents.
According to the change of the situations around the pressure-recording paper sheet, the following requirements to the performances of the pressure-sensitive recording paper sheet have been issued.
(1) The stability in preservation of the colour-developed image obtained by pressure-sensitive recording is improved.
(2) The recorded documents prepared by using the pressure-sensitive recording paper sheet are substantially odorless in placing it in storage.
(3) The pressure-sensitive recording paper sheet exhibit excellent colour-developing property.
It has been considered that it is extremely difficult to provide the pressure-sensitive recording paper sheet which satisfies the above-mentioned new requirements.
Ordinarily, the pressure-sensitive recording paper sheet is kept at room temperature for about 6 months before colour-development and for one to two years after colour-development, and the stability in preservation of the sheets for such a long time depends fundamentally on the chromogenic dye-precursor material, the colour-developer and the solvent of the chromogenic dye-precursor material. Particularly, concerning the solvent of the chromogenic dye-precursor material, a low-boiling solvent which evaporates just after performing its role as the carrier of the chromogenic dye-precursor material and as the medium of colour-developing reaction has been regarded as desirable.
However, since the pressure-sensitive recording paper sheet prepared by using such a solvent is not only poor in stability in preservation but also smell badly, such a solvent could not be said to be the solvent of chromogenic dye-precursor material for the pressure-sensitive recording paper sheet, which satisfies the above-mentioned requirements.
For instance, as the solvent of chromogenic dye-precursor material for the pressure-sensitive recording paper sheet, which has been hitherto used, the following compounds may be proposed.
Diisopropylnaphthalenes, 1-xylyl-1-phenylethanes, xylylphenylmethanes, partially hydrogenated terphenyls and butylbiphenyls.
Of the above-mentioned solvents of chromogenic dye-precursor material, partially hydrogenated terphenyl is excellent stability in preservation of the colour-developed image, however, the colour-developing activity of the solvent at a temperature of not higher than -5.degree. C. is extremely poor. In addition, although 1-xylyl-1-phenylethanes are excellent in colour-developing activity at low temperatures, it has a demerit of extremely strong in odor.
Other than the above-mentioned solvents, the following compounds have been proposed as the solvent of chromogenic dye-precursor material for the pressure-sensitive recording paper sheet. 1,2-Ditolylethanes (refer to U.S. Pat. No. 3,836,383), 1,1-Ditolylethanes (refer to U.S. Pat. No. 3,836,383) and 1,2-Ditolylpropanes (refer to Japanese Patent Application Laying-Open (KOKAI) No. 48-15611 (1973)).
As the synthetic method of 1,2-ditolylethanes disclosed in U.S. Pat. No. 3,836,383, the method following the Fiedel-Crafts reaction between toluene and 1,2-dichloroethane is adopted in general.
Since the synthesized product contains about 30% by weight of easily crystallizable 4,4'-dimethyl-1,2-diphenyl ethane which 4,4'-dimethyl-1,2-diphenylethane crystallizes out from the product at a low temperature of -5.degree. to 0.degree. C., the synthesized product is not preferable as the solvent of chromogenic dye-precursor material.
The trouble due to the precipitation of crystals occurs at the time of microcapsulation, of recording the pressure-sensitive recording paper sheet and of handling the solvent of chromogenic dye-precursor material.
For instance, on using the pressure-sensitive recording paper sheet at a low temperature of not higher than -5.degree. C., since a part of the solvent of chromogenic dye-precursor material crystallizes out from the solvent, the amount of the chromogenic dye-precursor material which dissolves in the solvent is reduced. As a result of the reduction, crystals of the chromogenic dye-precursor material are precipitated from the solvent. On carrying out the pressure-sensitive recording in such a situation, the recording is incompletely carried out because of the insufficient reaction between the chromogenic dye-precursor material and the colour-developer.
In addition, 1,1-ditolylethanes obtained by condensation of toluene with acetoaldehyde or by the reaction between toluene and 1,1-dichloroethane is poor in the stability in preservation of the colour-developed image.
Further, also 1,2-ditolylpropane obtained by the reaction between toluene and 1,2-dichloropropane or 1,2-ditolylpropanes obtained by the reaction between toluene and allyl chloride is poor in the stability in preservation of the colour-developed image.
Namely, with the remarkable propagation of the pressure-sensitive recording paper sheet and the change of the environment, an offer of a solvent of chromogenic dye-precursor material which is excellent in the stability in preservation of the colour-developed image and in the colour-developing activity at a low temperature of not higher than -5.degree. C. and is almost odorless, namely, a solvent to be put to practical use, has been keenly demanded.
On the other hand, concerning the coupling reaction by side-chain dehydrogenation, several methods have hitherto been known, for instance, a method of oxidative coupling of toluene at a temperature of 400.degree. to 600.degree. C. in the presence of a metal oxide catalyst has been known.
In Japanese Patent Application Laying-Open (KOKAI) No. 58-159428 (1983), a method of producing a dimer of toluene has been disclosed, wherein toluene is reacted in the presence of a certain quaternary ammonium salt in an aqueous solution of a peroxodisulfate. According to the above-mentioned method, although the dimer is obtained in a high yield, since it is necessary to use a large amount of the expensive quaternary ammonium salt, the method cannot be said to be an industrially desirable method.
The Japanese Patent Application Laying-Open (KOKAI) No. 58-159428 (1983) further discloses a process wherein the peroxodisulfate is regenerated by an electrochemical method.
However, in the case of electrochemically regenerating the peroxodisulfate, although the current efficiency is higher as the concentration of the sulfate ions is higher and it is desirable to maintain the high concentration of the sulfate ions, since the yield of the dimer is reduced by the presence of the sulfate ions at higher concentrations in the case of the coupling, the method cannot be said to be practical method. In addition, in the method, the addition of Ag.sup.+ or ions of transition metals to the reaction system for raising the yield of coupling causes a demerit of making the electrochemical regeneration difficult.
Namely, an offer of a method for liquid-phase coupling of ethylbenzene and xylene, by which the dimer can be obtained in a high yield and the peroxodisulfate can be electrochemically regenerated without using the expensive quaternary ammonium salt has been eagerly demanded.
As a result of the present inventors' studies for solving the above-mentioned problems, it has been found that by subjecting a mixture of ethylbenzene and xylene to "coupling by side-chain dehydrogenation" at a temperature of not less than 100.degree. C. in the presence of an aqueous solution of the peroxodisulfate, (1) diaryalkanes comprising 1,2-ditolylethanes, 1-tolyl-2-phenylpropanes and 2,3-diphenylbutane is obtained and (2) the thus obtained diarylalkanes has the excellent specificities as the solvent of chromogenic dye-precursor material for the pressure-sensitive recording paper sheet, is excellent in the stability in preservation of the colour-developed image and in the colour-developing activity at lower temperatures, and is almost odorless. Based on the above-mentioned findings, the present inventors have attained the present invention.
Namely, the objective of the present invention is to provide a solvent of chromogenic dye-precursor material for the pressure-sensitive recording paper sheet, the solvent comprising diarylalkanes which shows an excellent stability in preservation of the colour-developed image and an excellent colour-developing activity at a low temperature of not higher than -5.degree. C. and is almost odorless.
Furthermore, the objective of the present invention is to provide a process for producing diarylalkanes in a high yield by subjecting a mixture of ethylbenzene and xylene to "coupling by side-chain dehydrogenation" at a temperature of not less than 100.degree. C. in the presence of an aqueous solution of a peroxodisulfate.
In addition, another objective of the present invention is to provide a process for "coupling by side-chain dehydragenation", by which the dimer can be produced even in the presence of a high concentration of sulfate ions necessary for obtaining the high current efficiency in the case of electrochemically regenerating the peroxodisulfate from the aqueous solution after performing the reaction of "coupling by side-chain dehydrogenation".